nfs_congestion_kb is to control the max allowed writeback and in-commitpages. It's not reasonable for them to outnumber dirty and to-commitpages. So each of them should not take more than 1/4 dirty threshold.

Considering that nfs_init_writepagecache() is called on fresh boot,at the time dirty_thresh is much higher than the real dirty limit afterlots of user space memory consumptions, use 1/8 instead.

We might update nfs_congestion_kb when global dirty limit is changedat runtime, but whatever, do it simple first.